Amnesty International has backed the families of the Omagh bomb victims in their call for a cross-border inquiry into the 1998 explosion – the single biggest atrocity of the Northern Ireland Troubles.

The human rights organisation's support for an all-Ireland independent investigation comes before the release of a new report on Thursday that will conclude MI5, the FBI in the US and Garda special branch in the republic "starved" police in Northern Ireland of vital intelligence before the Real IRA massacre.

Twenty-nine people, including a woman who was pregnant with twins and her baby daughter, died after a 225kg (500lb) bomb exploded in the centre of the County Tyrone market town.

No one has been convicted in a criminal court in connection with the bombing in August 1998, which was part of the Real IRA's terror campaign aimed at destabilising Northern Ireland and preventing a political settlement.

The Omagh families have commissioned a private report compiled by the London law firm SBP and including security experts who, they say, have obtained sensitive intelligence material pointing to major failings by the security forces before the attack took place.

Amnesty International has challenged the US government along with Ireland and the UK to co-operate with any new inquiry into the bombing. It said there were concerns about the sharing of intelligence, or lack of it, in the runup to the attack.

Amnesty's director in Northern Ireland, Patrick Corrigan, said: "Fifteen years since the bomb ripped through Omagh, taking lives and causing injury, the families bereaved and those injured by the bomb are still left seeking the full truth about what happened that day and whether it could have been prevented.

"Beyond addressing the families' need for answers, there remains a broader public interest in establishing such an inquiry, in order to prevent such a tragedy recurring."

He added: "The families have had to suffer the indignity of being drip-fed information over the years, with new wounds opened each time and with none of the bombers ever being held criminally responsible.

"It is doubly sad that the bereaved families and those injured have now had to commission their own report as a result of the many partial investigations, each one of which opens up new questions."

The father of Aidan Gallagher, a young man killed in the blast said access to new intelligence files proved that the FBI, the Security Service and the Garda's crime and security branch all withheld vital information.

Michael Gallagher, who has campaigned since the attack for a cross-border public inquiry, said: "All good policing is based on intelligence especially prior intelligence before any criminal act is committed. In the case of the events running up to the Omagh bomb it is now clear that the police in the north were starved of information.

"The security forces in America, Britain and the republic had two key agents inside the Real IRA but did not share the information they were providing to the police in Northern Ireland."

The Omagh Support and Self Help Group also demanded a cross-border inquiry into the explosion.

"Should the state fail to do so it is our view that they will be failing in complying with their obligations under article 2(1) and article 3 of the European convention on human rights," it said. "This report is compiled from a significant amount of information, some of a sensitive nature, which has come into the possession of the families."

The report, released in Omagh on Thursday, focuses on the role of the two state agents who had infiltrated the Real IRA. They were David Rupert, a US national who was run by the FBI, and Paddy Dixon, a convicted criminal who procured cars in the Irish republic for the Real IRA for transporting car bombs into Northern Ireland throughout 1998.

Rupert arrived in Ireland in the mid-1990s and offered his services first to the Continuity IRA and later the Real IRA. The bankrupt businessman with links to Irish Americans had been targeted by the FBI as a potential agent. By the summer of 1997, he became involved with British intelligence. An FBI agent took him to a hotel in central London, where he was introduced to an MI5 officer who called himself Norman.

Norman advised Rupert not to pass all his information to the gardaí and provided him with a PO box address and a secret contact phone number.

MI5 urged him to offer intelligence to the Continuity IRA about British army and police bases in Northern Ireland. Posing as tourists awe-struck by the security installations, Rupert and his wife would stop at border-crossing points such as Aughnacloy and take pictures and videos of themselves. According to the Omagh Self Help and Support group new intelligence files indicate he was also aiding the Real IRA including sending bomb component parts from America. It was Rupert's testimony that helped convict the Real IRA founder Mickey McKevitt of organising acts of terrorism in 2003.

Dixon provided intelligence to a Garda handler on the Real IRA throughout 1998. He had a long-standing connection with a republican in County Dublin known as "the Long Fellow". His handler suggested – under orders from senior Garda command – that his old agent reactivate his relationship with "the Long Fellow", who owned a breaker's yard in south Dublin where Dixon's stolen cars were replated and huge explosive devices were secreted inside the vehicles.

Over the next seven months Dixon would give the Garda vital insight into the Real IRA terror machine. Between February and August 1998 Dixon gave the Irish police force inside information on at least nine separate Real IRA attacks culminating in the bomb at Omagh.

The Omagh families claim the "starving of intelligence" coming from Rupert and Dixon was designed to improve pair's credibility in the eyes of the Real IRA. But the Omagh campaigners claim the new report will show that this was a lethal error of judgment in relation to the terror group's final and most catastrophic attack in 1998.

The Real IRA was founded in November 1997 by McKevitt, the brother-in-law of the IRA hunger striker Bobby Sands. McKevitt led the breakaway faction from the Provisional IRA over the latter's move towards a peace strategy and opposition to a second ceasefire by the mainstream republican movement. The organisation he founded has split several times since the Omagh bomb and has now morphed into a new dissident republican terror alliance styling itself as the new IRA.